An on-vehicle radio antenna device generally includes: as shown in FIG. 10, an element 1 containing an antenna element for receiving radio waves; a case 2 for protecting an interior against bad weather; an amplifier board 3 for amplifying high frequency signals, which are received by the element 1; a cable (not shown) for transmitting the signals; abase 4 for holding components; and a nut 6 for fixing the base 4 to a vehicle panel 10. The amplifier board 3 is held on the base 4, a bottom face of the case 2 is closed with the base 4, and the amplifier board 3 is encased in the case 2. The element 1 is attached to an upper part of the case 2 (by screwing, for example, a female screw provided on the case 2 with a male screw provided on the element 1).
The base 4 of the on-vehicle radio antenna device has functions of holding the respective components, and fixing the antenna device to the vehicle panel 10 by means of the nut 6. Further, the base 4 has functions of conducting earth connection with respect to the vehicle panel 10, and so on. For this reason, conventionally, a die-cast product formed of metal has been a mainstream, because the die-cast product is produced from a material that can be formed in various shapes and has high strength, and through which electrical continuity can be achieved.
Generally, metal has a high specific gravity, and there has been a problem that the die-cast base formed of metal is heavy. Moreover, because metal has a high melting point, installation of molds is expensive, resulting in a high cost product. Further, because metal is electrically conductive, there is a risk that an accidental short-circuit may occur between the base and a cable for transmitting the received high frequency signals or a core wire of a power supply cable. On the other hand, there have been some structures in which a part of the base is formed of resin, which achieves weight reduction and cost reduction. However, the die-cast product is still a metallic part, and thus, cost reduction is not attained.
An example of the die-cast base formed of metal is disclosed in the below described Patent Document 1.